The present invention relates to a method for manufacturing a magnetic recording medium in which a flexible support (hereinafter referred to a web) made of plastic film, paper, metal foil, or the like, is coated with a coating liquid such as a magnetic liquid, a surface protecting liquid, or the like.
Conventional coating methods for applying a coating liquid onto a web in a process of manufacturing a magnetic recording medium include a roll-type coating method, a bead-type coating method, a curtain-type coating method, and an extrusion-type coating method. In all of those coating methods, a coating liquid is applied to a web across a width thereof narrower the actual width of the web so that, generally, uncoated portions in a range from several mm to several tens of mm remain along the two edges of the web.
Examples of extrusion-type coating apparatuses for applying a coating liquid onto a continuously running web with a coating head pressed against the web are disclosed in Japanese Unexamined Patent Publications Nos. Sho-57-84771, Sho-58-104666, Sho-58-109162, Sho-58-18069, Sho-58-18070, and Sho-60-78664, Japanese Patent Applications Nos. Sho-59-94657 and Sho-61-230173, Japanese Patent Publication No. Sho-45-1459. In these apparatuses, the web, on which lengthwise tension is exerted, is bent so that edge portions thereof are curved toward the coating head side so as to contact the coating head.
Due to this contact with the coating head, powder-like shavings are produced, which ultimately are a cause of dropout in the signal reproduced from the tape. Further, shavings deposited on the contact portion of the coating head with the web, etc., sometimes change the distance between the web and the coating head to thereby make the thickness of the coating liquid at the edge portions uneven.
It has been known that such an effect as caused in the coating head occurs at a doctor bar or a smoother, as disclosed in, for example, Japanese Unexamined Patent Publications Nos. Sho-51-140703, Sho-52-84242, and Sho-54-8646, Japanese Patent Publication Nos. Sho-48-11336 and Sho-60-53674.
In order to solve the foregoing problems, there has been proposed a method for providing a liquid lubricant between the coating head and web, in which, prior to application of the coating liquid, a solvent is applied onto the edge portions of the web to which no coating liquid is applied. Such a method is disclosed, for example, in Japanese Patent Unexamined Publications Nos. Sho-61-257268 and Hei-1-102736.
On the other hand, there has been proposed, as disclosed in, for example, Japanese Patent Unexamined Publication No. Sho-61-257263, Japanese Patent Application No. Sho-63-201996, Japanese Unexamined Patent Publication No. Hei-1-102735, another method in which portions of a coating head or the like corresponding to the edge portions of a web are cut away so as make the distance between the coating head and the web large.
In the conventional coating method in which a solvent is precoated to provide liquid lubrication, however, for example, in the case where a magnetic coating liquid is applied, there is a possibility that the coating liquid will be dissolved into the solvent, so that the quantity and range of coating are limited to thereby make it difficult to apply an extremely small amount of solvent onto the web edge portions. Further, external disturbances such as flapping, meandering, etc., due to running of the web make it difficult to apply the solvent while keeping the boundary fixed between the solvent and the coating liquid. As a result, there has been a problem that the coating liquid is mixed with the solvent so as to spread into the solvent. Further, there has been another problem that, by the above spreading, the magnetic coating liquid adheres to conveyer rolls used for supporting and running the web run, as a result of which the web is fouled by the dirty magnetic coating liquid, or a calender roll in the following surface treating step is also fouled, thus adversely affecting the calendering treatment and thereby degrading the magnetic recording characteristics of the medium.
In the method in which opposite end portions of a coating head are cut away, moreover, shavings are similarly generated by the cut edge portions on the opposite end portions of the coating liquid. While this method is effective at a portion of the coating liquid far away from the end portions of the web, these shavings adhere to the end portions. Further, because of the above-mentioned meandering of the web, it is extremely difficult to make the end portions of the coating layer formed by the coating head coincident with the cut-away positions of a doctor bar of a smoother disposed on the downstream side of o the coating head. Additionally, in the case where the web is thin, for example, not thicker than 30 .mu.m, and has particularly low rigidity, there has been a problem that the web is easily and greatly bent, so that the desired effects of cutting away end portions of the web cannot be obtained, even if cut-away portions are formed in the coating head or the like at its opposite end portions.
Further, the support is shaved when the running support comes into contact with the conveying system before and after coating, particularly during the calendering treatment, and the shavings so generated can result in undesired effects, for example, dropout during the reproduction of the magnetic recording medium.